1. Field of the Invention
The present invention relates to a liquid supply system, and more particularly to a liquid supply system suitable for use as a system for supplying ink to a recording head of an ink jet recording apparatus.
2. Description of the Related Art
The ink jet recording apparatus is an apparatus for forming a desired image on a recording medium, by spraying ink drops from a fine discharge port installed in a recording head and bumping the ink drops onto the recording medium. The ink jet recording apparatus has employed an ink mainly using dye. However, a recorded matter which has employed the ink using dye has insufficient light resistance and weathering resistance, and is unsuitable for use requiring the light resistance and the weathering resistance, such as articles intended for outdoor display. For this reason, an ink using pigment instead of dye is used. However, the pigment is not a dissolving system but a dispersing system, so that the ink using pigment occasionally causes the sedimentation of pigment particles in an ink tank.
The ink using pigment tends to cause a problem particularly in a structure of statically setting an ink tank accommodating a pigmented ink, in a main body of an ink jet recording apparatus of a serial type, which produces records while moving a recording head. Specifically, the ink using pigment occasionally causes a considerable sedimentation of pigment though depending on the frequency of use of a recording apparatus, the interval of use and the number of printed sheets. In a recoding apparatus having an out carriage tank separately arranged at a place apart from the recording head, the ink tank has occasionally a large capacity for the purpose of reducing the frequency of exchanging the tank. However, even in the tank with the large capacity, the pigment has occasionally sedimented.
When an ink tank is left alone in the state of being mounted on an ink jet recording apparatus for a long period of time, pigment particles slowly sediment in the ink tank. As a result of this, an inclination of the pigment particle concentration forms in a direction toward an upper part from the bottom of the ink tank. Accordingly, an excessively dark color layer forms in the bottom because of high pigment particle concentration, and an excessively light color layer forms in the upper part because of low pigment particle concentration. When ink is supplied from an ink tank having a structure of leading the ink from the bottom of the ink tank to a recording head, the ink is supplied first from the layer having the high pigment particle concentration and produces a printed matter with an excessively dark color. Therefore, there was a case where the recorded matter printed by using the ink in an early using stage of the ink tank showed an easily-visible difference of print density from that by using the ink in a latter stage of use.
The phenomenon is remarkable in a color printed matter in which an image is expressed by a difference of color density. In order to solve such a problem, Japanese Patent Application Laid-Open No. 2001-270131 and Japanese Patent Application Laid-Open No. 2001-293880 propose technologies. These patent documents disclose a technology of holding ink with an absorber in an ink tank, and arranging a tubular pipe having such a plurality of holes so as to communicate with an ink supply port of a tank, inside the tank. When a recording head sucks the ink, the ink existing in the pipe is sucked outside the tank by way of the supply port. The ink is sucked outside by way of a plurality of holes arranged in a vertical direction of the ink tank. In the structure, the ink of different concentrations is sucked from a plurality of the holes arranged in a vertical direction of the pipe in the tank, and is temporarily stored in the pipe. The ink is then supplied from the pool (pipe). Thereby, the structure uniformizes ink concentration and can alleviate the unevenness of pigment concentration due to sedimentation which occurs while being left for a long period of time in the ink tank.
In addition, U.S. Pat. No. 6,824,258 discloses a configuration in which an ink tank is connected to a recording apparatus, and air bubbles rise in a tank when ink is lead out. A structure is arranged in an upper space of the rising bubbles in the tank so as to disturb the rise of the bubbles. Then, the bubbles collide with the structure and agitate pigment particles in the tank. USP2004100540 discloses a configuration in which the ink is directly stored in the ink tank, and the air introduced from the outside of the tank circulates in a pipe installed in the ink tank and agitates pigment particles therein.
However, in configurations disclosed in these documents, pigmented ink in a tank is agitated by making use of an action such as ink sucking operated when a recording apparatus is recovered. The action includes, for instance, an operation of leading the sedimented pigment ink to the outside of the ink tank, a recovery operation carried out when removing bubbles in a recording head, and the operation for charging the ink into the recording head. Every time when these operations are performed, the ink in the tank is wastefully consumed though the pigment particles are agitated. Particularly, when the ink has a large difference of pigment concentration in the tank, the ink needs to be agitated many times before the concentration of the pigment of the ink in the tank is uniformized to the optimal pigment concentration. As a result of this, a large amount of the ink is inevitably drained outside the tank, so that the ink capable of being used originally in printing is wastefully drained.